1. Field of the Invention
The invention relates generally to heat exchange systems and is particularly directed to a compact gas-to-air heat exchanger and to a heating system incorporating the heat exchanger.
2. Description of Related Art
Broadly described, fuel-fired hot air heating systems generally comprise a combustion chamber, a heat exchange system for transferring heat from the hot products of combustion or flue gases to fresh or recirculated air, and a blower for moving the air through the heat exchange system and to the spaces which are to be heated. In the heating systems of interest herein, the flow of the combustion products is substantially less than the flow of air to be heated; the two flows may differ by an order of magnitude. For example, the weight ratio of combustion products to air may range from about 1:10 to about 1:12.
Known heat exchange systems for fuel-fired furnaces generally employ at least one shell-and-tube type heat exchanger or finned-tube heat exchanger, wherein the hot combustion products flow through the tubes and the air which is to be heated flows past the tubes in the shell region of the heat exchanger. Clam-shell type heat exchangers, wherein the flue gases flow inside a narrow passageway or channel defined by adjacent walls, are also common. In such prior heat exchangers, the combustion products flow through heat transfer conduits or passageways of relatively smaller total cross-sectional area as compared with the heat transfer conduits or passageways through which the air flows. Typically, the combustion products flow through "inside" passageways such as the inside of a tube of a shell-and-tube arrangement or the interior of the channel defined by the sealed walls of the clam-shell arrangement. In comparison therewith, the air flows through "outside" passageways such as the shell region between the exterior surfaces of the tubes in the tube-and-shell arrangement or the spaces adjacent the exterior surfaces of the walls of the clam-shell arrangement. The inside heat transfer coefficient for such arrangements is usually much less than the outside heat transfer coefficient, which results in the overall rate of heat transfer being controlled by the rate of heat transfer from the hot flue gases to the heat exchange surfaces.
Therefore, with conventional furnace heat exchangers, the amount of heat exchange surface area required to achieve a desired thermal efficiency is largely determined by the inside heat transfer coefficient and almost without any regard to the outside heat transfer coefficient. For these reasons, larger heat exchange surfaces are required with conventional shell-and-tube or clam-shell type furnace heat exchange arrangements than would be needed with a configuration having an inside heat transfer coefficient which is more nearly equal to the outside heat transfer coefficient. A furnace heat exchange configuration which requires less heat exchange surface area to achieve a desired thermal efficiency than conventional shell-and-tube or clam-shell type arrangements, would permit simpler and more compact furnace designs, and would, therefore, be highly advantageous.
Other furnace configurations used in residential applications include condensing-type furnaces such as that shown in U.S. Pat. No. 4,960,102 wherein a radial discharge burner is arranged as a primary heat exchanger/combustor and the combustion gases pass into a fin-and-tube secondary heat exchanger. The air to be heated passes over the exterior of the finned tubes of the secondary heat exchanger and then over the primary combustor/heat exchanger. U.S. Pat. No. 4,275,705 discloses a condensing-type furnace including a combined primary heat exchanger/combustor having several concentric exchanger runs and a fin-and-tube secondary heat exchanger for receiving combustion products. U.S. Pat. No. 4,941,452 which is owned by the assignee of this application discloses a pulse combustion space heater wherein air to be heated sequentially flows over a fin-and-tube secondary heat exchanger, a tailpipe and a combustion chamber.